The present invention relates generally to a wide-angle zoom lens equipped with a wide-angle end having a large angle of view, and more specifically to a wide-angle zoom lens for cameras using electronic imaging devices such as CCDs or C-MOSs, especially, a wide-angle zoom lens lending itself well to digital single-lens reflex cameras having a reflecting surface at its back focus.
There is now a growing market for single-lens reflex cameras using electronic imaging devices such as CCDs or C-MOSs. So far, zoom lenses used with conventional silver-halide film single-lens reflex cameras having an angle of view (2ω) of about 80° on the wide-angle side, an F-number of about 3.5 and a zoom ratio exceeding 3 have been often made up of a first lens group of negative power and a second lens group of positive power. As well known with some zoom lenses, focusing is carried out with the first lens group. Also, there has been proposed a zoom lens in which, at an angle of view (2ω) of about 85° or greater on the wide-angle side, the first lens group of negative power is divided into two subgroups with a varying spacing, as typically shown in patent publications 1 and 2.
Patent Publication 1
JP(A) 52-32342
Patent Publication 2
JP(A) 9-171139
Generally, single-lens reflex cameras using electronic imaging devices such as CCDs are smaller in the diagonal length of an image pickup plane than silver-halide film single-lens reflex cameras. In consideration of the so-called shading phenomenon, it is required to strike light beams on an image pickup plane substantially vertically from its center to its periphery. It is also required to allow for space for receiving a low-pass filter and so on. This causes an increase in the optical path length (back focus fb) ratio between the final surface of the taking lens needed for the diagonal length (the image-side surface of the lens nearest to the image side) and the image pickup plane. If this ratio is increased, there may then be obtained additional functions. On the other hand, the need for an increased back focus fb leads to the need for revamping a taking lens having a relatively short focal length.
However, there is not that more known about a zoom lens suitable for a single-lens reflex camera using an electronic imaging device and having an angle of view (2ω) of the order of 80° on the wide-angle side and a zoom ratio of higher than 3 yet capable of keeping high performance as near as object points at close range.